1. Field of the Invention
The present invention relates to an electric power converter apparatus having a plurality of semiconductor modules (i.e., modules each containing a plurality of interconnected semiconductor devices), with some of the semiconductor modules being connected in parallel.
2. Description of Related Technology
Types of electric power converter apparatus such as converters and inverters are known, which convert electrical power (e.g., voltage conversion and/or DC-AC conversion) by switching operation of semiconductor elements. The required levels of current which must be switched by such an electric power converter apparatus are becoming increasingly higher. This may be achieved by configuring an electric power converter apparatus using pluralities of semiconductor modules connected in parallel. In the prior art, from considerations of operating efficiency, it has been attempted to form layout and shapes of the conductors (i.e., current-carrying conductors) which connect to electrode terminals within each semiconductor module of a plurality of parallel-connected modules, such as to achieve uniform levels of controlled current through each of the semiconductor modules. This is described for example in Japanese patent application publication No. 2002-44960 (referred to in the following as reference document 1).
However, for example in the case of an electric power converter apparatus formed of a plurality of semiconductor modules stacked as an elongated array of modules, cooled by means of cooling tubes disposed in contact with the semiconductor modules, with a coolant medium circulating through the cooling tubes, variations will occur in the cooling effectiveness of the various semiconductor modules in accordance with their respective positions (upstream or downstream with respect to the flow of coolant medium). These variations result from differences in temperature occurring within the coolant medium, pressure losses within the coolant medium, etc.
The term “thermal resistance” of a semiconductor module, as used in the following description and in the appended claims, is to be understood as used in the specific sense of “relative cooling effectiveness” achieved for the is semiconductor module. That is to say, for any specific level of electrical power dissipated as heat by a semiconductor module, the operating temperature of the module increases in accordance with the thermal resistance of the module.
Thus, when a plurality of semiconductor modules are successively stacked and are cooled as described above by cooling tubes, with the cooling tubes and semiconductor modules successively alternating along the stacking direction (so that each module is contacted on opposing sides by cooling tubes), with a coolant medium flowing from an input port into the cooling tubes and being discharged from an outlet port, the respective values of thermal resistance of the semiconductor modules will successively increase in accordance with increasing distance of the modules from the intake port, along the stacking direction of the modules (i.e., will successively increase from the upstream side to the downstream side of the coolant flow path). Temperature variations will thereby arise between the respective semiconductor modules. That is to say, there will be a bias towards successively increasing temperature values of the semiconductor modules, from the upstream side to the downstream side. Hence it is not possible to operate all of the semiconductor modules efficiently while ensuring that none of the modules have electrical characteristics adversely affected by excessively high operating temperature.
It is an objective of the present invention to overcome the above problem, by providing an electric power converter apparatus having a plurality of semiconductor modules whereby such a bias of temperature values of respective semiconductor modules can be prevented. More efficient operation of the electric power converter apparatus can thereby be achieved.
To achieve the above objective, according to a first aspect, the invention provides an electric power converter apparatus which includes at least one set of (plurality of) semiconductor modules which are connected in parallel. Each of the semiconductor modules comprises a plurality of electrode terminals (in general, two electrode terminals) for transferring a flow of electric current through the semiconductor module. Respectively corresponding ones of the electrode terminals of the semiconductor modules are electrically connected via corresponding connecting leads (referred to herein as branch conductor leads) to a common branch point.
The basic characterizing feature of the present invention is that corresponding branch conductor leads of the various semiconductor modules are configured with respectively different values of impedance, such that the impedance values successively increase in accordance with increased values of thermal resistance of the semiconductor modules.
In that way, temperature differences between the semiconductor modules can be reduced, since the level of current flow through a semiconductor module having a relatively high value of thermal resistance is correspondingly decreased (thereby reducing the electrical power dissipated within the semiconductor module), due to a relatively increased impedance of a connecting lead (or connecting leads) carrying current of the semiconductor module.